22 After this short and simple discourse concerning the Trinity, the third book makes further progress, sure though slow. Citing the greatest instances of His power, it brings within the range of faith's understanding that saying, in itself beyond our comprehension, I in the Father and the Father in Me, which Christ utters concerning Himself. Thus truth beyond the dull wit of man is the prize of faith equipped with reason and knowledge; for neither may we doubt God's Word concerning Himself, nor can we suppose that the devout reason is incapable of apprehending His might.
Source: On the Trinity (New Advent)